The primary objective of this study is to measure the efficacy of solifenacin succinate (VESIcare) in reducing the mean number of micturitions per 24 hour period in Parkinson's disease (PD) patients as measured by voiding diaries. [ Time Frame: 12 weeks ] [ Designated as safety issue: No ]

Women of child-bearing potential must use a reliable method of contraception.

Must be experiencing symptoms of overactive bladder according to the ICS definition of a minimum voiding 8 or more times/24 hours and a daily average of at least 1 episode of urgency and/or urinary incontinence (urge incontinence predominately as measured by 3IQ diary) per 24 hours during a 3-day micturition diary period. Patients must have documentation of OAB within the last 6 months.

The patient must have evidence of PSA less than or equal to 4 (males only) within the last 12 months (obtained from primary care physician).

The patient must have had a bladder scan within six months of the screening visit. This scan uses ultrasound technology to measure residual fluid levels in the bladder after urination. This scan must document post void residual of 200 mls or less. A bladder scan printout or a note documenting these findings must be provided before baseline.

Clearance from the patient's internist or primary care health provider who has examined the patient within the last 6 months.

Exclusion Criteria:

Any illness that in the investigator's opinion preclude participation in this study.

Pregnancy or lactation.

Concurrent participation in another clinical study.

Dementia or other psychiatric illness that prevents the patient from giving informed consent (Mini Mental Status Exam scores less than 27).

Legal incapacity or limited legal capacity.

History of prostate cancer or Transurethral resection of the prostate (TURP) (males only).

Presence of severe renal disease. BUN 50% greater than normal (normal BUN levels should be within a range of 5 to 20 mg/ d L and creatinine between .7 and 1.4 mg/ d L). Labs within the past 12 months will be requested from the patient's health care provider or urologist. If labs are not available within this time-frame or if results are abnormal, labs will be obtained as part of the screening visit.